This invention relates to trawl doors, also known as trawl boards. Trawl doors are devices used in the fishing industry to pull and spread a fishing net while being towed by a vessel. In use, a net is attached to each of two trawl doors which are in turn each connected to a tow cable arrangement attached to the vessel. When towed, the trawl doors keep the mouth of the net spread open. Typically, trawl doors are shear members made either of wood or metal and are designed to exert a desired spreading force when pulled through the water. Present trawl doors, and in particular wood trawl doors, have considerable natural weight or require the addition of ballast to make them assume a desired depth in the water when towed. These heavy or weighted doors contribute to increased power requirements and fuel consumption of the towing vessel which is of course undesirable. Accordingly, it is desirable that the depth of operation be adjustable without having to add or remove ballast. Further, it is also desirable that the lateral or spreading force exerted by the trawl door on the net be easily adjustable with minimum effect on drag.
One known trawl door is disclosed in U.S. Pat. No. 3,410,014. That trawl door includes two V-shaped partially overlaping shear members joined along their edges and having their concave sides facing each other. An angularly adjustable elevator is mounted in the passageway formed between the two shear members to control the depth of operation. While this trawl door has less drag in the water and thus effects a decrease in power and fuel consumption of the towing vessel and does provide for controlling the depth of operation, there is no means for controlling the lateral net spreading force generated by the door. Further, although the two shear member construction does reduce weight of the door over solid wood doors, the door still has considerable weight and drag in the water and further improvement is desirable.